Conventionally, concerning the voice recognition in an onboard navigation system or in a hands-free phone, an echo canceler has been known which can recognize or transmit to a party via a phone only voices of a talker by eliminating, from voices input to a microphone, music and voices output from an onboard speaker or speakers (see Patent Documents 1 and 2, for example).
As disclosed in the foregoing Patent Documents 1 and 2, an echo canceler of a type that generates an elimination signal of echoes mixed into a microphone from echo sound sources via speakers by processing reference signals input from the echo sound sources with adaptive filters must perform echo cancellation processing for each of the sound sources causing echoes (referred to as an “echo sound source” from now on).
For this reason, considering the maximum number of echo sound sources supposed, such as 6 channels-8 channels, a processor with high processing capacity must be mounted. Furthermore, in a condition having only the number of echo sound sources less than the maximum number of echo sound sources supposed, since it becomes unnecessary to carry out some of the processing of the adaptive filters, the echo cancellation processing lead to waste, and wasteful allocation of processor resources occurs.
In addition, in the adaptive filters, the number of taps required varies depending on environments in which the voice recognition system or hands-free phone is used, and when determining the number of taps on the assumption of the worst environment, a processor with very high processing capacity is required because the echo cancellation effect increases with an increase of the number of taps.
For this reason, in a voice recognition system for AV equipment, a technique has been known conventionally which reduces the processing amount of a processor by always making the reference signals of the echo sound sources monaural (see Patent Document 3, for example). Since the technique can always reduce the number of the echo sound sources to one, it can prevent the echo cancellation processing from causing waste by the reduction of the number of the echo sound sources.